1. Field of the Invention
The present invention relates to an image forming apparatus wherein an image formed on an image bearing member is transferred onto a movable body. More particularly, it relates to an image forming apparatus wherein an image formed on an image bearing member is transferred onto a transfer sheet borne on a movable body such a transfer drum to form an image on the transfer sheet.
2. Related Background Art
As a way for obtaining an image by transferring an image formed on an image bearing member onto a transfer sheet borne on a transfer drum, there has been proposed a method wherein a color image is formed on a transfer sheet by superimposing a plurality of different color toner images on the transfer sheet by repeating steps for forming the toner image on an image bearing member by charging, exposure and development and for transferring the toner image onto the transfer sheet whenever the toner image is formed. Such a color image forming apparatus is disclosed in the German Patent No. 2607727 and the Japanese Patent Laid-open Application No. 50-50935 and is put into practical use.
By the way, in such conventional color image forming apparatuses, the transfer sheet is wound around a transfer drum constituted by a pair of ring members, a connection member connecting between the ring members, and a high resistive sheet or film covering an opening defined by the ring members and the connection member. However, in such a transfer drum, independent charge means are required as an absorb charger, a transfer charger, a separation charger and an electricity removal charger for the transfer sheet, and there is a problem regarding the strength and handling of the transfer drum, since the opening defined by the ring members and the connection member is closed by the high resistive film.
On the other hand, to eliminate the above problem, as disclosed in EP-A-548803, the inventors have proposed a transfer device comprising a transfer drum constituted by laminating an elastic layer and a high resistive film on a conductive cylinder.
In such a transfer device, the transferring operation by using the transfer drum constituted by laminating the elastic layer and the high resistive film on the conductive cylinder is effected by forming a nip between the image bearing member and the transfer drum, and the transferring efficiency is greatly influenced upon a width of the nip. The nip width is determined by an compressed amount t of the elastic layer of the transfer drum against the image bearing member, i.e. a difference between a radius of the transfer drum at the nip between the image bearing member and the transfer drum and a radius of the transfer drum when the drum is not abutted against the image bearing member. Thus, an outer diameter of the transfer drum must be formed with high accuracy.
That is to say, if the compressed amount t of the elastic layer is too small, the nip width will be insufficient to adequately urge the transfer sheet on the transfer drum against the image bearing member, thereby causing the poor transferring. On the other hand, if the compressed amount t is too great, the urging pressure of the transfer sheet against the image bearing member will be increased excessively, thereby causing the void in fine lines in the image or the peeling of the transfer sheet from the transfer drum due to the great flexure of the transfer sheet at the nip. Further, since the outer diameter of the transfer drum is great, a moving speed of the transfer sheet borne on the transfer drum is increased, thereby causing the elongation of the image and the deviation in color registration.
On the other hand, hardness of the elastic layer must be low in order to reduce the urging pressure between the transfer drum and the image bearing member at the nip, thereby preventing the void. Accordingly, eventually, the workability of the elastic layer is worsened, with the result that it is very difficult to form the outer diameter of the transfer drum with high accuracy and to make the surface of the transfer drum uniform and smooth.